1. Field of the Invention
The present invention relates to a laser diode driver, and more particularly to a high power, high pulse repetition frequency, pulsed laser diode driver.
2. Description of the Prior Art
Generally, laser drivers are mainly classified into gas lasers, solid-state lasers and semiconductor lasers. Although generating a high output power, gas lasers and solid-state lasers are bulky, heavy and expensive. Furthermore, they exhibit a deterioration in efficiency. On the other hand, semiconductor lasers are compact, light, inexpensive and very efficient. By virtue of such advantages, the utilization of the semiconductor lasers have recently been on increasing trend.
The conventional high power pulsed laser driver has a very high circuit impedance. Due to the severe impedance mismatch between the driver circuit (high impedance) and the laser array (extremely low impedance), rather than the electrical energy is used to operate the laser array, most of it is lost in the form of heat. However, for the operation of the high power semiconductor laser, current beyond the threshold level should be supplied to the semiconductor laser. Therefore, the lost energy as heat has to be compensated by increasing the pulse biasing voltage.
Therefore, the conventional high power, pulsed driver is designed with much high power capability. This requires even higher power semiconductor switch.
In the conventional high power, pulsed driver, as the lost energy is increased, the required power capability of the semiconductor switch goes up steeply. As a result, the capabilities of the laser driver such as the rise and fall times, pulsewidth and pulse repetition frequency (PRF) of the output laser pulse deteriorates rapidly, while steeply increasing the size and weight of the driver.
A critical parameter in the semiconductor laser diode operation is the supplied current level. At low current levels, namely, below threshold current level, laser diodes generate some spontaneous emission without laser output (laser light). As the current level increases, diode lasers pass a threshold where the population in the laser diode medium becomes inverted and laser action begins.
Therefore, below threshold current, very little laser light is emitted and its emission efficiency is very low. Once the current level passes the threshold, the light output rises steeply.
High power laser diodes, called the laser diode stripes or laser arrays, are produced by fabricating a large number of laser diodes on a single substrate. The laser output power level is proportional to the numbers of the laser diodes in the laser array. Obvious advantages of this fabrication technique are low manufacturing cost, mass production, miniaturization, and high reliability. The disadvantage is an extremely low on-state device resistance (much less than 1 ohm).
Because the laser arrays are fabricated by connecting numerous numbers of forward biased p-n junction device, namely, laser diode, in parallel, the on-state resistance of the laser arrays goes down as the numbers of the laser diodes in the array increase. Typically, on-state resistance of the high power laser arrays is in the range of a few ohms to less than 0.01 ohm. Meanwhile, as the output power level of the laser arrays increases (number of the laser diodes in the array gets bigger), the threshold current level for these lasers rises steeply.
The modulation scheme for the pulsed high power laser operation is a direct modulation in which the laser light is modulated by controlling the current flow into the laser array. For the high power, high PRF, pulsed laser operation, very high current pulse at high PRF has to be generated by the laser driver and delivered to an extremely low impedance load (laser array).
The capabilities of conventional high power, pulsed laser drivers primary depend on the capabilities of the high power semiconductor switches (such as silicon controlled rectifier (SCR), power field effect transistor (power FET), IGBT (insulated gate bipolar transistor) and power bipolar transistor).
The conventional high power pulsed laser driver, utilizing a circuit topology in which the high voltage capacitor is pulse biased and then the electrical energy is discharged by turning on the power semiconductor switch, has a very high circuit impedance. Due to the severe impedance mismatch between the driver circuit (high impedance) and the laser array (extremely low impedance), rather than the electrical energy is used to operate the laser array, most of it is lost in the form of heat. The amounts of generated heat are so large that a fan has to be installed to remove this heat from the driver. Further, (since the operation of a laser array requires certain current level), the lost energy has to be compensated by increasing the biasing voltage.
As the power handling capability of the conventional laser driver goes up, other capabilities (such as the rise and fall times, pulsewidth and PRF of the output laser pulse) of the laser driver deteriorate rapidly, while steeply increasing the size and weight of the driver.
As a result, the conventional high power, pulsed drivers are heavy and very bulky, compared to the laser arrays, and their capabilities such as the rise time, fall time and PRF are severely limited.
Beside the peak laser output power capability, other important and critical parameters of the pulsed laser driver are modulation speed (high PRF), pulsewidth, efficiency, weight and compactness. There is no available pulsed laser driver producing high peak power light pulse with narrow pulsewidth at high PRF while maintaining high efficiency, light weight and compactness.